The invention concerns a process for the control of inking in a printing machine, a printing plant suitable for the carrying out of the process and a measuring apparatus for the generation of the control data for such a printing plant.
In continuous printing the control of inking is the most important possibility of affecting the impression of the image. It is performed by visual evaluation or by means of a densitometric analysis of color measuring fields printed with the image. An example of the latter is described in German Patent Publication OS 27 28 738, which corresponds to U.S. Pat. No. 4,200,932.
More specifically, the color impression of an image printed in an offset printing machine is best regulated by control of the inking, i.e. control of the physical thickness of the color inks applied to the sheet of paper onto which the image is printed. Ink layer thicknesses can be controlled within certain given (narrow) limits, whereby thicker layers result in more saturated color impression or higher (full-tone) color densities, and vice versa. Full-tone color densities and thicknesses of ink layers are directly related and these terms are even often used synonymously. For the definition of color densities please refer to the literature on the subject, such as the International Standard Publication ISO 5/3-1984, "Photography-Density Measurements-Part 3: Spectral Conditions", First Edition-Aug. 15, 1984, International Organization for Standardization, which is hereby incorporated by reference.
Control of image impression is usually performed by means of special color measuring fields (color test fields, color test strip, color measuring strip) printed together with the image. The measuring fields are opto-electrically scanned and the color density values thereby obtained are compared with desired reference values, e.g. obtained from a so-called "O.K." sheet. Examples of color measuring fields and suitable (scanning) denistometers are described for example, in U.S. Pat. Nos. 3,995,958; 4,494,875; and 4,505,589 as well as in the many references cited in these patent specifications.
The control of the ink thicknesses is effected on the basis of the deviations of the measured color density values from the desired reference density values in such a way as to minimize these deviations. An example of an automatic closed-loop ink control system of this kind is described in the aforementioned U.S. Pat. No. 4,200,932. Other similar systems such as that shown in FIG. 1A have been on the market for many years, one of them being the "Heidelberg Speedmaster" system.
Offset printing presses generally work on a zonal basis, i.e. the printing width is divided into e.g. 32 printing zones each of which is controlled independently from the others (at least as far as the present invention is concerned). By means of a control panel various control functions of the printing press can be performed. For example, the control panel can be fed with color density deviation data (control data) and regulate the ink control elements in the printing press on the basis of these data in a manner such that prints produced after the corresponding regulating step have lower or--ideally--no density deviation as compared to desired reference color densities. The control panel can be fed with a suitable set of color density deviations such that one deviation is provided for each printing ink and for each printing zone (e.g. 3.times.32 density deviations in case of a three color printing press having 32 printing zones).
It has been discovered in actual practice, however, that the control of inking on the basis of densitometric measurements alone is often insufficient. Thus, it happens frequently that in the case of a setting for equal full-tone densities, appreciable color differences appear between proofs or proof substitutes, respectively, and production runs. These perceived color differences must then be corrected manually by the interactive adjustment of the ink controls. The causes of such differences in printed color may be found in the generally different production processes for proofs/substitute proofs and for production runs and in the color differences of the materials used. Furthermore, in the case of constant ink density printing and in particular full-tone density printing, constancy of the ink impression is not assured because variations of the tone value occur as the result of soiling of the rubber blanket or of other effects.
Thus, there is a need in the prior art for more suitable input control data for known printing control systems in order to achieve more satisfying ink control.